Living in a browning environment: Effects on Daphnia's growth and fatty acid pattern

To assess whether different concentrations of dissolved organic matter (DOM) rich in humic substances (HS) affect growth rates, reproduction and fatty acids (FA) composition of Daphnia pulex under bright light, we performed microcosms controlled laboratory trials. Experiments were run using two microalgae as food source, Cryptomonas erosa and Scenedesmus acutus at four DOM additions (0 mg C L-1, 15 mg C L-1, 30 mg C L-1, and 60 mg C L-1) under unlimited food concentrations. Results show that daphnids' growth rates were significantly higher feeding on Scenedesmus than on Cryptomonas, but were not significantly affected by the HS-rich-DOM concentrations. In contrast, clutch sizes increased with increasing concentrations of HS-rich-DOM, with nonsignificant differences between both food treatments. As expected, FA in daphnids were mainly impacted by the food. Irrespectively of food, the content of total FA and especially saturated FA were lower in the treatments with the highest addition of HS-rich-DOM. In the Scenedesmus trial, certain highly unsaturated FA (HUFA) susceptible to UV degradation could only be observed in the treatments with higher concentrations of HS-rich-DOM (e.g., eicosapentaenoic acid [EPA] and arachidonic acid [ARA] at 30, and EPA, ARA and docosahexaenoic acid [DHA] only at 60 mg L-1). In the Cryptomonas trial, DHA levels increased with increasing concentrations of HS-rich-DOM. Our results show that FA mediated food quality impacted how HS-rich-DOM affected Daphnias' growth. As the high dietary content in HUFA probably caused daphnids lower growth rates under UV exposure, there seems to be a trade-off between beneficial and potential detrimental effects of feeding on a diet rich in HUFA.